METHODS FOR REMOTE DETECTION OF MINERAL COMPOSITION IN THE ALUNITE-JAROSITE GROUP FOR APPLICATION TO MARS

In situ and remotely sensed data have indicated the presence of minerals from the alunite-jarosite group at multiple locations across Mars. Because these minerals are typically deposited from acidic aqueous solutions, their occurrence offers an opportunity to determine environmental conditions at the time of their formation. Minerals in the alunite-jarosite group have the basic formula [(K, Na, H₃O⁺)(Al, Fe)₃(SO₄)₂(OH)₆] with considerable compositional variability. If information can be extracted about the chemical composition in martian samples, it would allow even tighter constraints to be placed on depositional conditions. We synthesized over 100 alunite-jarosite group minerals with varying chemical composition, identified the products by XRD and SEM-EDS analyses, and selected the purest samples for characterization by Raman and visible-near infrared spectroscopy. Previous work had investigated Al-Fe mixing in the natroalunite-natrojarosite series (McCollom et al., Am. Mineral., 99, 948-964, 2014), so our more recent studies sought to expand this to consider K and hydronium (H₃O⁺) substitution for Na. The results indicate that XRD peaks and VNIR and Raman spectral features vary systematically with chemical composition, and that these features can be used to place constraints on compositions using remote detection methods. Our findings can assist in the characterization of these alunite-jarosite group minerals during both current and upcoming Mars missions, such as Mars 2020 with its SHERLOC Raman spectrometer.